752 PLANT RESPONSE 



stress, there is complete recovery ; and the same wire, 

 when torsioned beyond the hmits of elasticity, does not 

 recover, but remains permanently strained. In the same 

 way, we have seen that the effect of stimulus on a living 

 tissue is to induce a molecular derangement, from which 

 there is complete recovery, with a concomitant recovery 

 of all the physiological properties, if the derangement have 

 been not too great ; but, with excessive or long-continued 

 stimulus, the limit of physiological elasticity is exceeded, and 

 a permanent physiological differentiation is thus induced. 



The same causes, moreover, which initiated the primitive 

 differentiation may now act to induce further a series of com- 

 plex movements. Thus we first see plagiotropic dorsi- 

 ventrality induced in the creeping stem of Cticurbita, by the 

 unilateral action of sunlight ; and then the diurnal periodicity 

 of light and darkness acting on this already differentiated 

 organ to cause a periodic swing, which increases with repeti- 

 tion. In this we have, as has been pointed out, the first 

 stage in the evolution of the nyctitropic movement. Now, it 

 is quite possible that this nyctitropic movement may be 

 found, at leastMn some cases, to subserve the advantage of 

 the plant ; yet it would not be true to say that it was 

 evolved for the purpose of such advantage. Indeed, we have 

 to guard ourselves carefully against being led, by this theory 

 of the final advantage of the plant, into an argument in a 

 circle. Assuming any given movement to be advantageous 

 to the plant, we have first to determine the nature of the 

 mechanism by which it is produced, and secondly to find out 

 what was the exciting cause, and what the character of the 

 conditions under which it first arose. If we are not guided 

 in our inquiry by such considerations, we are liable to be 

 misled in our inferences. One such example was seen in the 

 general belief that a certain specific sensibility resides in the 

 root-tip, by which it is endowed with the faculty of moving 

 away from rough surfaces, which might have been injurious 

 to it. So far from its having any such peculiar faculty, how- 

 ever, we found that when a red-hot wire was presented to it 



